OK, so I'm pretty handy with my Powerbook. In the past, I've replaced 2 different hard drives, a DVD-R drive and a few other things. In short, I've torn my Mac apart and put it back together successfully on numerous occasions. By this latest problem has really stumped me. I've read up on everything I can find on the Internet and changed out many parts, but still my LCD backlight is dead. Here is what I've done so far...

1) First assumed it was the Inverter board, so I ordered a replacement, opened up the screen (man, that was a bear) and installed it. Still, no backlight.2) Next, assumed it was the inverter cable. Repeated above process with new cable. No light.3) Next, assumed it might be the backlight itself, so I ordered one (complete with compatible 2-pin connector so no soldering needed). I didn't actually install it (which is an even bigger bear than lid removal), but I did plug it into the inverter output to test it out and... you guessed it... no love.4) I figured that somewhere along the way, I burned out the replacement inverter board that I bought (OK, OK, I put a meter across the outputs... oops), so I bought ANOTHER replacement inverter. Plugged it in, powered it up... still no backlight (tried both original bulb and new replacement bulb).

At this point, I am completely stumped. I checked the input voltages to the inverter board and the seem OK, although I can't find anything on the Internet that states exactly what they should be. But they aren't zero (at least not all of them). Here's what they are...

So, I'm in for about 90 bucks so far in parts and the thing is still dead. Oh yeah and, by the way, I know it's the backlight because I can see the faint images on the screen AND the laptop works fine when connected to an external monitor. I already have a newer MacBook Pro that I use for everyday stuff, but it would really be nice to get the old G4 working again. Anyone have any suggestions about where to go from here?

John

Author:

johnmc22 [ Tue May 18, 2010 6:28 pm ]

Post subject:

Re: Dead LCD Backlight on Powerbook G4 Aluminum 17"

OK, I solved my own problem… sort of. The bottom line was that the logic board was NOT outputting any voltage for the “dim control” input to the Inverter board. According to some other info found on the Internet, the “dim control” pin should have a voltage ranging from 0 to +4 volts depending on the dimming desired (where 0V = no light, 4V = max light). Since the logic board wasn’t outputting any voltage on that pin, the backlight remained off. So, since I had no intention of buying a new logic board for such an old computer, I decided to just manually put +4V on the pin. Here’s how I did it…

So, it was the DIM CONTROL that was dead from the logic board. I had both 12v and 5v available to me at the input connector to the Inverter board so, at first I figured I’d just connect the 5v to DIM CONTROL. At first it worked but, after about 1 minute, the backlight would shut off, presumably because the Inverter was being “overdriven” and it has some kind of overcurrent shutdown circuitry. So, I went to plan B and, though experimentation, found that a 3K ohm resistor placed between pin 4 and pin 5 brought the voltage on the DIM CONTROL to +3.8V which kept the backlight bright without it shutting down. See pictures below.

Now, for the bad news. Not only will the screen not dim (which I can live with), but it also won’t shutoff when going into sleep mode (the display goes away, but the backlight remains lit). For the record, it DOES shutoff when the computer is ShutDown. But, this poses yet another challenge. Since I generally only use my computer from home while it is plugged in, I may not worry about this one for now. But, for many people, this might be a showstopper. None the less, I did get the backlight working.